Fixing apparatus of induction heating type for fixing image formed on sheet

ABSTRACT

A fixing apparatus includes an induction heating coil configured to heat a heat generating member including a conductive heating element, a boosting circuit configured to boost a DC voltage obtained by rectifying AC power, a switching element configured to input a DC voltage boosted by the boosting circuit and to supply a high-frequency current to the induction heating coil, a driving circuit configured to drive the switching element, a temperature detection unit configured to detect a temperature of the heat generating member, and a control unit configured to control power supplied to the induction heating coil by controlling a boosting ratio of the boosting circuit and a driving frequency of the switching element by the driving circuit so that the temperature detected by the temperature detection unit reaches a target temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/615,879 filed Nov. 10, 2009, which claims priority from JapanesePatent Application No. 2008-288944 filed Nov. 11, 2008, all of which arehereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power supply circuitry for an inductiveheating element. A fixing apparatus of the induction heating type may beincorporated in an image forming apparatus, and the power supplycircuitry may be used to supply power to an inductive heating element insuch fixing apparatus.

2. Description of Related Art

The image forming apparatus generally contains a fixing device forfixing a toner image transferred to a recording material. As the fixingdevice, a heating type device using a ceramic heater or a halogen heaterhas conventionally been used in many cases. Recently, an electromagneticinduction heating type device has begun to be used (refer to JapanesePatent Application Laid-Open No. 2000-223253).

FIG. 12 illustrates a simple frequency control method employed for powercontrol of a power supply unit, which supplies power to a fixing deviceof the induction heating type. In steps 4001 and 4002, detected power Pis compared with target power Po. In the case of P>Po, then in step4005, the frequency is increased by a predetermined value fa. In thecase of P<Po, then in step 4004, the frequency is decreased by apredetermined value fb. In the case of P=Po, then in step 4003, thefrequency is maintained.

FIG. 13 illustrates a simple frequency control method employed fortemperature control of the fixing device. In steps 5001 and 5002, adetected temperature T is compared with a target temperature To. In thecase of T>To, then in step 5005, the frequency is increased by apredetermined value fa. In the case of T<To, then in step 5004, thefrequency is decreased by a predetermined value fb. In the case of T=To,then in step 5003, the frequency is maintained.

FIG. 14 illustrates a relationship between a driving frequency f andpower P. As illustrated in FIG. 14, maximum power Pmax is supplied to acoil at a resonance frequency f1. Characteristically, supplied power isreduced when the frequency changes to a high-frequency side or alow-frequency side relative to the resonance frequency f1. Thus, it ispossible to achieve power control by controlling the driving frequency fwithin a frequency range fh above the resonance frequency f1, in whichrange the power-frequency characteristic has a slope. It is alsopossible to control the power by controlling the driving frequencywithin a frequency range f1 below the resonance frequency f1.

More specifically, in a frequency control system, to reduce power, thedriving frequency for a switching element, which is used to supply powerto the coil, is set higher than the resonance frequency. However, whenthe driving frequency becomes higher than the resonance frequency,switching losses of the switching element may increase. Losses areparticularly conspicuous when a large-power operation is performed in astate in which the driving frequency deviates from the resonancefrequency.

Moreover, in a DC voltage control system for controlling power onlybased on a change in DC voltage supplied to the switching element, botha boosting circuit and a de-boosting circuit are required, thus leadingto a great increase in production cost and circuit size.

SUMMARY OF THE INVENTION

It is desirable to provide power supply circuitry capable of reducinglosses of a switching element during a large-power operation whilesuppressing an increase in cost and size of the circuitry.

According to an aspect of the present invention, a fixing apparatusincludes an induction heating coil configured to heat a heat generatingmember including a conductive heating element, a boosting circuitconfigured to boost a DC voltage obtained by rectifying AC power, aswitching element configured to input a DC voltage boosted by theboosting circuit and to supply a high-frequency current to the inductionheating coil, a driving circuit configured to drive the switchingelement, a temperature detection unit configured to detect a temperatureof the heat generating member, and a control unit configured to controlpower supplied to the induction heating coil by controlling a boostingratio of the boosting circuit and a driving frequency of the switchingelement by the driving circuit so that the temperature detected by thetemperature detection unit reaches a target temperature. The controlunit is configured to selectively execute a first control mode forcontrolling the power supplied to the induction heating coil by changingthe driving frequency of the switching element within a range offrequencies equal to or higher than a predetermined frequency and asecond control mode for controlling the power supplied to the inductionheating coil by changing the boosting ratio of the boosting circuitwithin a range of ratios equal to or higher than a predeterminedboosting ratio.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a sectional diagram illustrating a configuration of an imageforming apparatus according to an exemplary embodiment of the presentinvention.

FIG. 2 is a sectional diagram illustrating a configuration of a fixingdevice.

FIG. 3 is a circuit diagram illustrating a configuration of a powersupply unit of the fixing device.

FIG. 4 illustrates a relationship between a driving frequency of a coiland power.

FIG. 5 illustrates a relationship between an output voltage of aboosting circuit and power.

FIG. 6 is a control flowchart for a fixing device according to a firstexemplary embodiment of the present invention.

FIG. 7 is a control flowchart for a fixing device according to a secondexemplary embodiment of the present invention,

FIG. 8 illustrates a relationship among a driving frequency, an outputvoltage of a boosting circuit and power according to the second exemplarembodiment.

FIG. 9 is a table illustrating a relationship among power, an outputvoltage of a boosting circuit and a driving frequency according to athird exemplary embodiment of the present invention.

FIG. 10 illustrates a relationship in changes between the output voltageof the boosting circuit and the driving frequency according to the thirdexemplary embodiment.

FIG. 11 is a control flowchart for a fixing device according to thethird exemplary embodiment.

FIG. 12 is a power control flowchart based on frequency control of aconventional fixing device.

FIG. 13 is a temperature control flowchart based on the frequencycontrol of the conventional fixing device.

FIG. 14 illustrates a relationship between a driving frequency of a coiland power.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

FIG. 1 is a sectional diagram illustrating a configuration of a colorimage forming apparatus according to a first exemplary embodiment of thepresent invention. The apparatus is an image forming apparatus that usesan electrophotography process.

After uniform charging of photosensitive members 1 a to 1 d by primarycharging units 2 a to 2 d, exposure units 3 a to 3 d irradiate thephotosensitive members 1 a to 1 d with laser beams modulated accordingto an image signal to form electrostatic latent images on thephotosensitive members 1 a to 1 d. Then, developing units 4 a to 4 ddevelop toner images. Primary transfer units 53 a to 53 d transfer thetoner images on the four photosensitive members 1 a to 1 d to anintermediate transfer belt 51 in a superimposed manner. Further,secondary transfer units 56 and 57 transfer the toner images torecording paper P. Cleaners 6 a to 6 d collect toner left untransferredon the photosensitive members 1 a to 1 d. An intermediate transfer beltcleaner 55 collects toner left untransferred on the intermediatetransfer belt 51. A fixing device 7 fixes the toner image transferred tothe recording paper P, so that a color image is obtained. The fixingdevice 7 has a configuration of the electromagnetic induction heatingtype.

FIG. 2 is a sectional diagram illustrating the configuration of thefixing device of the electromagnetic induction heating type. A fixingbelt 72 is a metal belt serving as a heating member, which includes aconductive heating element, and its surface is covered with a rubberlayer of 300 μm. The fixing belt 72 rotates around rollers 73 and 74 ina shown arrow direction. A fixing belt 75 rotates around rollers 76 and77 in a shown arrow direction. An induction heating coil 71 is locatedin a coil holder 70 opposite the fixing belt 72, which includes aconductive heating element. An AC current flows through the coil 71 togenerate a magnetic field, so that the conductive heating element of thebelt 72 generates heat by itself. Thermistors 78 a, 78 b and 78 c arelocated in contact with center, rear, and front sides of the belt 72 ina depth direction to detect a temperature of the belt 72. Thethermistors 78 a, 78 b and 78 c are resistors that exhibit resistancevalues higher as a temperature is lower. In the fixing device 7, an ACcurrent flowing through the coil 71 is increased or decreased so thatthe temperature detected by the center thermistor 78 a reaches 190° C.,which is a target temperature. Upper and lower pads 90 and 91 applypressure of about 40 kg weight on the belts 72 and 75.

FIG. 3 is a block diagram illustrating a configuration of a power supplyunit 100, which supplies power to the fixing device 7 of the inductionheating type. An AC power source 500 supplies power to the power supplyunit 100. An AC voltage from the AC power source 500 is rectified by adiode bridge 101, and the rectified voltage is smoothed by a filtercapacitor 102. A resonance capacitor 105 constitutes a resonance circuitwith the coil 71. A boosting circuit 108 boosts a DC voltage rectifiedby the diode bridge 101, and its boosting ratio is variable. Forexample, the boosting ratio changes within a range of 1 to 3. First andsecond switching elements 103 and 104 control power supplied to the coil71. A switch driving circuit 112 drives the switching elements 103 and104 with switch driving signals 121 and 122. The boosting circuit 108,switching elements 103, 104, switch driving circuit 112 and capacitor105 form part of a driving signal generator which supplies coil drivingsignals to the coil 71. A control unit 113 controls the boosting circuit108 and the switch driving circuit 112. A power detection circuit 111detects input power from the AC power source 500. A temperaturedetection circuit 114 detects a temperature of the belt 72 based onsignals from the thermistors 78 a to 78 c. The control unit 113determines power to be supplied to the coil 71 based on a detectionresult from the power detection circuit 111 and a detection result fromthe temperature detection circuit 114, and determines drivingfrequencies of the switch driving signals 121 and 122 output from theswitch driving circuit 112 and a boosting ratio of the boosting circuit108 so that power supplied to the coil 71 reaches the determined power.The switching elements 103 and 104 are alternately turned ON/OFFaccording to the switch driving signals 121 and 122 to supply coildriving signals (a high-frequency current) to the coil 71.

With the above-described configuration, the power supply unit 100operates in a frequency control mode when using a first power range inwhich the boosting circuit 108 operates at a boosting ratio of 1, i.e.,Vo=Vi, and operates in a voltage control mode when using a second powerrange higher than the first power range.

FIG. 4 illustrates a relationship between frequencies of the switchdriving signals 121 and 122 of the switching elements 103 and 104 outputfrom the switch driving circuit 112 and power supplied to the coil 71.

In a characteristic curve when the boosting ratio of the boostingcircuit 108 is maintained at 1, i.e., Vo=Vi, power P supplied to thecoil 71 is set equal to reference power Pr (P=Pr) when a frequency f ofthe driving signal is a resonance frequency f1. When the frequency f ofthe driving signal is increased from f1 to f2, the power P is set to P4lower than the reference power Pr. When the frequency of the drivingsignal is increased more and more, the power P can be reduced more. Toincrease the power P more than the reference power Pr, the boostingratio of the boosting circuit 108 is increased while the frequency f ofthe driving signal is maintained at f1. In other words, increasing theboosting ratio as Vo=V3, V2, and V1 (V3<V2<V1) in order results in anincrease in power supplied to the coil 71 as P3, P2, and P1. Thus, thepower P can be increased without increasing switching losses.

FIG. 5 illustrates a relationship between an output voltage Vo of theboosting circuit 108 and power P when frequencies f of the drivingsignals 121 and 122 are equal to the resonance frequency f1.

Thus, in the present exemplary embodiment, two modes of power control,frequency control mode and voltage control mode, are set, and eachcontrol mode is selectively executed. Specifically, the frequencycontrol mode is a mode (first control mode) for controlling power to besupplied by changing the driving frequency of the switching elementwithin a range of frequencies equal to or higher than a predeterminedfrequency in a state where the boosting ratio of the boosting circuit108 is maintained at a predetermined boosting ratio. The voltage controlmode is a mode (second control mode) for controlling power to besupplied by changing the boosting ratio of the boosting circuit 108within a range of ratios equal to or higher than a predeterminedboosting ratio in a state where the driving frequency of the switchingelement is maintained at a predetermined frequency.

FIG. 6 is a flowchart illustrating power control for the fixing device 7executed by the control unit 113. In the present exemplary embodiment,it is presumed that a temperature T of the center of the belt 72, atwhich the thermistor 78 a is located, is controlled to a targettemperature To.

First, in step 999, the control unit 113 initially sets a mode of powercontrol to the frequency control mode at the time of starting anoperation. The initial setting of the mode to the frequency control modeis for the purpose of gradually increasing power from a low power stateto increase the temperature of the belt 72 at the time of startingcontrol. In step 1000, the control unit 113 determines whether thecontrol mode is the voltage control mode at a point of this time. Whendetermining that the mode is the frequency control mode, then in steps1001 and 1002, the control unit 113 compares the detected temperature Tbased on an output of the thermistor 78 a with the target temperatureTo. In the case of T>To, then in step 1007, to decrease the temperatureof the belt 72, the control unit 113 increases the frequency by apredetermined value fb. The processing then returns to step 1000. In thecase of T<To, the control unit 113 is required to increase thetemperature of the belt 72. Then in step 1003, the control unit 113determines whether a value obtained by decreasing the frequency by apredetermined value fa is higher than a resonance frequency f1, in otherwords, whether the value satisfies “f−fa≧f1”. In the case of f−fa≧f1,then in step 1006, to increase the temperature of the belt 72, thecontrol unit 113 decreases the frequency by the predetermined value fa.The processing then returns to step 1000. If not f−fa≧f1, then in step1005, the control unit 113 sets the frequency to f1. In step 1008, thecontrol unit 113 switches the mode of power control from the frequencycontrol mode to the voltage control mode. The processing then returns tostep 1000. In steps 1001 and 1002, in the case of T=To, the control unit113 maintains the set frequency f.

When determining in step 1000 that the mode of power control is thevoltage control mode at a point of this time, then in steps 1011 and1012, the control unit 113 compares the detected temperature T based onthe output of the thermistor 78 a with the target temperature To. In thecase of T<To, the control unit 113 is required to increase thetemperature of the belt 72. Then in step 1017, the control unit 113determines whether power P supplied to the coil 71 is less than upperlimit power Pmax. If it is not the case that P<Pmax, the control unit113 maintains an output voltage Vo of the boosting circuit 108 as it is.The processing then returns to step 1000. In the case of P<Pmax, then instep 1019, the control unit 113 sets the boosting ratio to increase theoutput voltage Vo of the boosting circuit 108 by a predetermined valueVb. The processing then returns to step 1000. In the case of T>To, thenin step 1013, the control unit 113 determines whether a value obtainedby decreasing the output voltage Vo of the boosting circuit 108 by apredetermined value Va is lower than an input voltage Vi of the boostingcircuit 108, in other words, whether the value satisfies “Vo−Va<Vi”. Inthe case of Vo−Va<Vi, then in step 1016, the control unit 113 sets theboosting ratio to decrease the output voltage Vo of the boosting circuit108 by the predetermined value Va. The processing then returns to step1000. If it is not the case that Vo−Va<Vi, then in step 1015, thecontrol unit 113 sets Vo=Vi (boosting ratio to 1). Then, in step 1018,the control unit 113 switches the mode of power control from the voltagecontrol mode to the frequency control mode. The processing then returnsto step 1000. In the case of T=To, the control unit 113 maintains theoutput voltage Vo of the boosting circuit 108 as it is. The processingthen returns to step 1000.

For example, assuming that an inductance of the fixing device 7 is 40 μHand a capacity of the resonance capacitor 105 is 1 μF, the resonancefrequency f1 is about 25 kHz. When a voltage of the commercial powersource 500 is 100 V, in the configuration of the present exemplaryembodiment, the voltage Vi is about 140 V and the reference power Pr atthis time is 500 W. Thus, the power supply unit 100 operates in thevoltage control mode where the driving frequency is maintained at 25 kHzwhen supplying a power larger than 500 W, and operates in the frequencycontrol mode (driving frequency 25 kHz or higher) where the outputvoltage of the boosting circuit 108 is maintained at 140 V whensupplying a power smaller than 500 W.

As described above, when supplying a relatively large power (>500 W)which requires high efficiency, changing the boosting ratio whiledriving the switching element at the resonance frequency enables areduction in losses of the switching element. When supplying arelatively small power (≦500 W), changing the driving frequency of theswitching element enables power control without needing any de-boostingcircuit.

Configurations of an image forming apparatus and a power supply unitaccording to a second exemplary embodiment of the present invention aresimilar to those of the first exemplary embodiment. FIG. 7 is aflowchart illustrating power control executed by the control unit 113 inthe second exemplary embodiment. In the second exemplary embodiment, asin the case of the first exemplary embodiment, it is presumed that atemperature T of the center of the belt 72, at which the thermistor 78 ais located, is controlled to a target temperature To. Also, the powersupplied when the boosting ratio of the boosting circuit 108 is set to apredetermined boosting ratio (boosting ratio 1) and the drivingfrequency of the switching element is set to a predetermined frequency(resonance frequency f1) is used as a reference power Pr.

First, in step 1997, the control unit 113 detects a voltage of thecommercial power source 500. In step 1998, the control unit 113 sets apower Pa and a power Pb, which are used as references for switchingbetween the voltage control mode and the frequency control modeaccording to a voltage detection value. In other words, the power Pa isset to a first predetermined power lower than the reference power Pr.The power Pb is set to a second predetermined power larger than thereference power Pr. A relationship among Pa, Pb, and Pr is Pa<Pr<Pb asillustrated in FIG. 8. Next, in step 1999, the control unit 113initially sets the mode of power control to the frequency control mode.The initial setting of the mode to the frequency control mode is for thepurpose of gradually increasing power from low power at the time ofstarting control. In step 2000, the control unit 113 determines whetherthe mode of power control is the voltage control mode at a point of thistime. When determining that the mode is not the voltage control mode butthe frequency control mode, then in steps 2001 and 2002, the controlunit 113 compares a detected temperature T with the target temperatureTo. In the case of T>To, then in step 2007, the control unit 113increases the frequency by a predetermined value fb. The processing thenreturns to step 2000. In the case of T<To, then in step 2003, thecontrol unit 113 compares power P supplied to the coil 71 with the setvalue Pa. In the case of P<Pa, then in step 2006, the control unit 113decreases the frequency by a predetermined value fa. The processing thenreturns to step 2000. In the case of P≧Pa, then in step 2005, thecontrol unit 113 sets the frequency to f=f1. In step 2008, the controlunit 113 switches the mode of power control to the voltage control mode.In step 2002, if not T<To, in other words, in the case of T=To, thecontrol unit 113 maintains the set frequency f as it is. The processingthen returns to step 2000.

On the other hand, when determining in step 2000 that the mode of powercontrol is the voltage control mode at a point of this time, then insteps 2011 and 2012, the control unit 113 compares the detectedtemperature T with the target temperature To. In the case of T<To, thenin step 2017, the control unit 113 determines whether power P is lessthan upper limit power Pmax. If it is not the case that P<Pmax, thecontrol unit 113 maintains the output voltage Vo of the boosting circuit108. The processing then returns to step 2000. In the case of P<Pmax,then in step 2019, the control unit 113 increases the output voltage Voof the boosting circuit 108 by the predetermined value Vb. Theprocessing then returns to step 2000. In the case of T>To, then in step2013, the control unit 113 compares power P with the set value Pb. Inthe case of P>Pb, then in step 2016, the control unit 113 decreases theoutput voltage Vo of the boosting circuit 108 by the predetermined valueVa. The processing then returns to step 2000. In the case of P≧Pb, thenin step 2015, the control unit 113 sets V0=Vi. In step 2018, the controlunit 113 switches the mode of power control to the frequency controlmode. If it is not the case that T>To in step 2012, in other words,T=To, the control unit 113 maintains the output voltage Vo of theboosting circuit 108. The processing then returns to step 2000.

For example, assuming that the inductance of the fixing device 7 is 40μH and the capacity of the resonance capacitor 105 is 1 μF, theresonance frequency f1 is about 25 kHz. When the voltage of thecommercial power source 500 is 100 V, the voltage Vi is about 140 V, andthe power Pr at a point of this time is 500 W in the configuration ofthe fixing device 7 according to the present exemplary embodiment. Inthis case, the power Pa is set to 470 W, and the power Pb is set to 530W.

When the voltage of the commercial power source 500 is 120 V, the powerPr is 720 W. In this case, the power Pa is set to 690 W, and the powerPb is set to 750 W.

Configurations of an image forming apparatus and a power supply unitaccording to a third exemplary embodiment of the present invention aresimilar to those of the first and second exemplary embodiments.

In the third exemplary embodiment, the control unit 113 has a tablestoring data as illustrated in FIG. 9. The stored data is divided into aplurality of sets of data numbered from 1 to 8. Each set of datacorresponds to a different power P (P1 to P7 or 0) and indicates arelationship between the output voltage Vo of the boosting circuit 108and the driving frequency f applicable at the power concerned. Thecontrol unit 113 selects one of the data sets (combination of outputvoltage Vo (boosting ratio) and driving frequency f) in the tableaccording to a difference between the target temperature and thedetected temperature of the fixing device 7. FIG. 10 is a graphicrepresentation of the relationship indicated in the table illustrated inFIG. 9.

By stepping through the data sets numbered 1 to 3 of the table, i.e.,powers P1 to P3, the control unit 113 performs control in the voltagecontrol mode, which maintains the frequency at f=f1 and changes thevoltage Vo. In data set number 4, i.e., power Pr, the control unit 113maintains the driving frequency at f=f1 and the voltage Vo=Vi. Bystepping through the data sets numbered 5 to 7, i.e., powers P5 to P7,the control unit 113 performs control in the frequency control mode,which maintains the voltage Vo=Vi and changes the driving frequency f.In other words, with power Pr set as a boundary, the control unit 113selects the voltage control mode when power higher than Pr is necessary,and the frequency control mode when power lower than Pr is necessary. Inthe present exemplary embodiment, there are eight combinations of Vo andf. However, more segmentation is available between the data numbers 1and 8.

FIG. 11 is a flowchart illustrating power control executed by thecontrol unit 113 according to the third exemplary embodiment. In thethird exemplary embodiment, as in the case of the first and secondexemplary embodiments, it is presumed that the temperature T of thecenter of the conductive heating element 72, at which the thermistor 78a is located, is controlled to a target temperature To.

When control is started, in step 2997, the control unit 113 detects thevoltage of the commercial power source 500. In step 2998, the controlunit 113 sets a table of combinations of output voltages Vo and drivingfrequencies f of the boosting circuit as illustrated in FIG. 9. Morespecifically, the control unit 113 determines whether the commercial ACpower source is a 100 V or 200 V system. The control unit 113 sets atable for 100 V in the case of the 100 V system, and a table for 200 Vin the case of the 200 V system. The control unit 113 may set differenttables depending on countries or regions where the image formingapparatus is installed. Next, in step 2999, the control unit 113 sets adata set number, indicating a combination of the output frequency Vo ofthe boosting circuit and the driving frequency f, to 8. The data setnumber 8 indicates a power stop state. In step 3000, the control unit113 compares the detected temperature T with the target temperature To.In the case of T>To, then in step 3006, the control unit 113 determineswhether a data number X set at this point in time (hereinafter referredto as a current data set number) is 8, in other words, a stop state. Ifthe data set number is 8, the control unit 113 maintains the data setnumber X as it is. The processing then returns to step 3000. If the dataset number is not 8, the processing proceeds to step 3007. To decreasepower to be supplied to the induction heating coil 71, the control unit113 changes the combination to that of Vo and f set by a number higherby one than the current data set number X. Thus, when the fixing device7 exceeds the target temperature, the control unit 113 may sequentiallyincrease the data number X by repeating steps 3000, 3006, 3007, 3000, .. . , and even X=8 (power stop state) may be set.

If it is not the case that T>To in step 3000, the processing proceeds tostep 3001. If T<To in step 3001, then in step 3002, the control unit 113determines whether the current data set number X is 1, in other words,maximum power setting. If the data set number X is 1, the control unit113 maintains the data set number as it is. The processing then returnsto step 3000. If in step 3002 the data set number X is not 1, theprocessing proceeds to step 3004. In step 3004, to increase power to besupplied to the induction heating coil 71, the control unit 113 changesthe combination to a combination of Vo and f set by a number lower byone than the current data set number X. Thus, when the fixing device 7is cold at the time of turning-ON of power or the like, the control unit113 may sequentially decrease the data set number X by repeating steps3000, 3001, 3002, 3004, 3000, . . . , until X=1 is reached. If it is notthe case that T<To in step 3001, the control unit 113 maintains the datanumber X as it is. The processing then returns to step 3000.

As described above, when supplying a relatively large power whichrequires high efficiency, changing the boosting ratio while driving theswitching element with the resonance frequency enables changes in powerwhile reducing losses of the switching element. When supplying arelatively small power, changing the driving frequency of the switchingelement enables power control without needing any de-boosting circuit.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

What is claimed is:
 1. A fixing apparatus comprising: an inductionheating coil configured to heat a heat generating member including aconductive heating element; a boosting circuit configured to boost a DCvoltage obtained by rectifying AC power; a switching element configuredto input a DC voltage boosted by the boosting circuit and to supply ahigh-frequency current to the induction heating coil; a driving circuitconfigured to drive the switching element; a temperature detection unitconfigured to detect a temperature of the heat generating member; and acontrol unit configured to control power supplied to the inductionheating coil by controlling a boosting ratio of the boosting circuit anda driving frequency of the switching element by the driving circuit sothat the temperature detected by the temperature detection unit reachesa target temperature, wherein the control unit is configured toselectively execute a first control mode for controlling the powersupplied to the induction heating coil by changing the driving frequencyof the switching element within a range of frequencies equal to orhigher than a predetermined frequency and a second control mode forcontrolling the power supplied to the induction heating coil by changingthe boosting ratio of the boosting circuit within a range of ratiosequal to or higher than a predetermined boosting ratio, and wherein thecontrol unit is configured to select one of the first control mode andthe second control mode based on the temperature detected by thetemperature detection unit, the boosting ratio, and the drivingfrequency.
 2. The fixing apparatus according to claim 1, wherein thecontrol unit is configured to maintain the boosting ratio of theboosting circuit at the predetermined boosting ratio in the firstcontrol mode, and to maintain the driving frequency of the switchingelement at the predetermined frequency in the second control mode. 3.The fixing apparatus according to claim 1, wherein the control unit isconfigured to execute the first control mode at the time of starting anoperation of the fixing apparatus.
 4. A fixing apparatus comprising: aninduction heating coil configured to heat a heat generating memberincluding a conductive heating element; a boosting circuit configured toboost a DC voltage obtained by rectifying AC power; a switching elementconfigured to input a DC voltage boosted by the boosting circuit and tosupply a high-frequency current to the induction heating coil; a drivingcircuit configured to drive the switching element; a temperaturedetection unit configured to detect a temperature of the heat generatingmember; and a control unit configured to control power supplied to theinduction heating coil by controlling a boosting ratio of the boostingcircuit and a driving frequency of the switching element by the drivingcircuit so that the temperature detected by the temperature detectionunit reaches a target temperature, wherein the control unit isconfigured to selectively execute a first control mode for controllingthe power supplied to the induction heating coil by changing the drivingfrequency of the switching element within a range of frequencies equalto or higher than a predetermined frequency and a second control modefor controlling the power supplied to the induction heating coil bychanging the boosting ratio of the boosting circuit within a range ofratios equal to or higher than a predetermined boosting ratio, andwherein in a state where the first control mode is selected, when thetemperature detected by the temperature detection unit is lower than thetarget temperature, if a value obtained by decreasing a drivingfrequency that is set when the temperature is detected by thetemperature detection unit by a predetermined value is lower than thepredetermined frequency, the control unit is configured to switch fromthe first control mode to the second control mode.
 5. A fixing apparatuscomprising: an induction heating coil configured to heat a heatgenerating member including a conductive heating element; a boostingcircuit configured to boost a DC voltage obtained by rectifying ACpower; a switching element configured to input a DC voltage boosted bythe boosting circuit and to supply a high-frequency current to theinduction heating coil; a driving circuit configured to drive theswitching element; a temperature detection unit configured to detect atemperature of the heat generating member; and a control unit configuredto control power supplied to the induction heating coil by controlling aboosting ratio of the boosting circuit and a driving frequency of theswitching element by the driving circuit so that the temperaturedetected by the temperature detection unit reaches a target temperature,wherein the control unit is configured to selectively execute a firstcontrol mode for controlling the power supplied to the induction heatingcoil by changing the driving frequency of the switching element within arange of frequencies equal to or higher than a predetermined frequencyand a second control mode for controlling the power supplied to theinduction heating coil by changing the boosting ratio of the boostingcircuit within a range of ratios equal to or higher than a predeterminedboosting ratio, and wherein in a state where the second control mode isselected, when the temperature detected by the temperature detectionunit is higher than the target temperature, if a value obtained bydecreasing a boosting ratio that is set when the temperature is detectedby the temperature detection unit by a predetermined value is lower thanthe predetermined boosting ratio, the control unit is configured toswitch from the second control mode to the first control mode.
 6. Afixing apparatus comprising: an induction heating coil configured toheat a heat generating member including a conductive heating element; aboosting circuit configured to boost a DC voltage obtained by rectifyingAC power; a switching element configured to input a DC voltage boostedby the boosting circuit and to supply a high-frequency current to theinduction heating coil; a driving circuit configured to drive theswitching element; a temperature detection unit configured to detect atemperature of the heat generating member; and a control unit configuredto control power supplied to the induction heating coil by controlling aboosting ratio of the boosting circuit and a driving frequency of theswitching element by the driving circuit so that the temperaturedetected by the temperature detection unit reaches a target temperature,wherein the control unit is configured to selectively execute a firstcontrol mode for controlling the power supplied to the induction heatingcoil by changing the driving frequency of the switching element within arange of frequencies equal to or higher than a predetermined frequencyand a second control mode for controlling the power supplied to theinduction heating coil by changing the boosting ratio of the boostingcircuit within a range of ratios equal to or higher than a predeterminedboosting ratio, wherein in a state where the first control mode isselected, when the temperature detected by the temperature detectionunit is lower than the target temperature, and the power to be suppliedto the induction heating coil is set higher than first predeterminedpower, the control unit is configured to switch from the first controlmode to the second control mode, and wherein the first predeterminedpower is power smaller than the power supplied to the induction heatingcoil when the boosting ratio of the boosting circuit is equal to thepredetermined boosting ratio and the driving frequency is equal to thepredetermined frequency.
 7. A fixing apparatus comprising: an inductionheating coil configured to heat a heat generating member including aconductive heating element; a boosting circuit configured to boost a DCvoltage obtained by rectifying AC power; a switching element configuredto input a DC voltage boosted by the boosting circuit and to supply ahigh-frequency current to the induction heating coil; a driving circuitconfigured to drive the switching element; a temperature detection unitconfigured to detect a temperature of the heat generating member; and acontrol unit configured to control power supplied to the inductionheating coil by controlling a boosting ratio of the boosting circuit anda driving frequency of the switching element by the driving circuit sothat the temperature detected by the temperature detection unit reachesa target temperature, wherein the control unit is configured toselectively execute a first control mode for controlling the powersupplied to the induction heating coil by changing the driving frequencyof the switching element within a range of frequencies equal to orhigher than a predetermined frequency and a second control mode forcontrolling the power supplied to the induction heating coil by changingthe boosting ratio of the boosting circuit within a range of ratiosequal to or higher than a predetermined boosting ratio, wherein in astate where the second control mode is selected, when the temperaturedetected by the temperature detection unit is higher than the targettemperature, and the power to be supplied to the induction heating coilis set lower than second predetermined power, the control unit isconfigured to switch from the second control mode to the first controlmode, and wherein the second predetermined power is power larger thanthe power supplied to the induction heating coil when the boosting ratioof the boosting circuit is equal to the predetermined boosting ratio andthe driving frequency is equal to the predetermined frequency.
 8. Afixing apparatus comprising: an induction heating coil configured toheat a heat generating member including a conductive heating element; aboosting circuit configured to boost a DC voltage obtained by rectifyingAC power; a switching element configured to input a DC voltage boostedby the boosting circuit and to supply a high-frequency current to theinduction heating coil; a driving circuit configured to drive theswitching element; a temperature detection unit configured to detect atemperature of the heat generating member; and a control unit configuredto control power supplied to the induction heating coil by controlling aboosting ratio of the boosting circuit and a driving frequency of theswitching element by the driving circuit so that the temperaturedetected by the temperature detection unit reaches a target temperature,wherein the control unit is configured to selectively execute a firstcontrol mode for controlling the power supplied to the induction heatingcoil by changing the driving frequency of the switching element within arange of frequencies equal to or higher than a predetermined frequencyand a second control mode for controlling the power supplied to theinduction heating coil by changing the boosting ratio of the boostingcircuit within a range of ratios equal to or higher than a predeterminedboosting ratio, and wherein the control unit is configured to increasethe power to be supplied to the induction heating coil when thetemperature detected by the temperature detection unit is lower than thetarget temperature, to decrease the power to be supplied to theinduction heating coil when the temperature detected by the temperaturedetection unit is higher than the target temperature, to select thefirst control mode when the power to be supplied is smaller than thepredetermined power, and to select the second control mode when thepower to be supplied is larger than the predetermined power.
 9. Thefixing apparatus according to claim 8, wherein the predetermined poweris power supplied to the induction heating coil when the boosting ratioof the boosting circuit is equal to the predetermined boosting ratio andthe driving frequency is equal to the predetermined frequency.
 10. Thefixing apparatus according to claim 8, further comprising a tableconfigured to store data indicating a relationship between the boostingratio and the driving frequency corresponding to the power to besupplied, wherein in the data of the table, the boosting ratio and thedriving frequency are determined according to the first control modewithin a range in which the power to be supplied is smaller than thepredetermined power, and are determined according to the second controlmode within a range in which the power to be supplied is larger than thepredetermined power.
 11. A fixing apparatus configured to fix an imageformed on a sheet, the fixing apparatus comprising: an inductive heatingelement; a driving signal generating unit configured to generate drivingsignals to be supplied to the inductive heating element; a temperaturedetection unit configured to detect a temperature of an object heated bythe inductive heating element; and a control unit configured to controla voltage and a frequency of the driving signals in dependence upon thedetected temperature so as to tend to maintain the object at a targettemperature, the control unit being switchable between a first controlmode, in which the voltage of the driving signals is maintainedsubstantially at a predetermined voltage and the frequency of thedriving signals is changed, and a second control mode in which thefrequency of the driving signals is maintained substantially unchangedand the voltage of the driving signals is changed to a voltage greaterthan or equal to the predetermined voltage.
 12. The fixing apparatusaccording to claim 11, wherein in the second control mode the drivingsignal generating unit is operable to generate the driving signals byboosting an input voltage and in the first control mode the drivingsignal generating unit is operable to generate the driving signalswithout boosting the input voltage.
 13. The fixing apparatus accordingto claim 11, wherein the driving signal generating unit is configured toform a resonant circuit with the inductive heating element, and in thesecond control mode the frequency of the driving signals is maintainedat or close to a resonant frequency of the resonant circuit.
 14. Afixing apparatus configured to fix an image formed on a sheet, thefixing apparatus comprising: an inductive heating element; a drivingsignal generating unit configured to generate driving signals to besupplied to the inductive heating element; a temperature detection unitconfigured to detect a temperature of an object heated by the inductiveheating element; and a control unit configured to control a voltage anda frequency of the driving signals in dependence upon the detectedtemperature so as to tend to maintain the object at a targettemperature, the control unit being switchable between a first controlmode, in which the voltage of the driving signals is maintainedsubstantially unchanged and the frequency of the driving signals ischanged to a frequency greater than or equal to a predeterminedfrequency, and a second control mode in which the frequency of thedriving signals is maintained substantially at the predeterminedfrequency and the voltage of the driving signals is changed.
 15. Thefixing apparatus according to claim 14, wherein in the second controlmode the driving signal generating unit is operable to generate thedriving signals by boosting an input voltage and in the first controlmode the driving signal generating unit is operable to generate thedriving signals without boosting the input voltage.
 16. The fixingapparatus according to claim 14, wherein the driving signal generatingunit is configured to form a resonant circuit with the inductive heatingelement, and in the second control mode the frequency of the drivingsignals is maintained at or close to a resonant frequency of theresonant circuit.
 17. A fixing apparatus configured to fix an imageformed on a sheet, the fixing apparatus comprising: an inductive heatingelement; a driving signal generating unit configured to generate drivingsignals to be supplied to the inductive heating element; a temperaturedetection unit configured to detect a temperature of an object heated bythe inductive heating element; and a control unit configured to controla voltage and a frequency of the driving signals in dependence upon thedetected temperature so as to tend to maintain the object at a targettemperature, the control unit being switchable between a first controlmode, in which the voltage of the driving signals is maintainedsubstantially unchanged and the frequency of the driving signals ischanged, and a second control mode in which the frequency of the drivingsignals is maintained substantially unchanged and the voltage of thedriving signals is changed, wherein the control unit is operable toswitch from the first control mode to the second control mode when thedetected temperature is less than the target temperature and the powersupplied is less than a first reference power, and is further operableto switch from the second control mode to the first control mode whenthe detected temperature is greater than the target temperature and thepower supplied is greater than a second reference power greater than thefirst reference power.
 18. The fixing apparatus according to claim 17,wherein the first reference power is less than a power supplied when thedriving signals have the predetermined voltage and the predeterminedfrequency, and the second reference power is greater than the powersupplied when the driving signals have the predetermined voltage and thepredetermined frequency.